The Doctor (Matt Smith) tries to get everyone to Rio, but nothing quite goes as planned. They arrive in a small Welsh village where the Doctor immediately senses that the ground isn't quite right. There is a drilling rig nearby that has reached a depth of 21 kilometers when odd things begin to happen. A local village man has disappeared and soon after Amy (Karen Gillan) is swallowed into the ground. The Doctor realizes that the drilling has disturbed an ancient reptilian race that has lived deep underground for thousands upon thousands of years and who are determined to protect themselves against the 'apes' who live on the surface.